The embryo of perennial Medicago sativa L. and annual M. scutellata (L.) Mill. have similar growth stages, but the perennial embryo is smaller and its rate of growth is slower than that of the annual by about 3 days. Transfer cells in the suspensor and embryo sac of late heart stages suggest different major pathways of nutrient flow in the two species. Transfusion tissue at the base of the embryo sac in the ovule of M. scutellata may facilitate solute transport and promote rapid embryo growth. Plastids in the suspensor cells of heart and late heart stages of the two species contain a dense matrix, membrane-bounded plastid vacuoles, starch, and a dense core. The plastid core in M. sativa has stacked tiers of straight tubules about 24 nm in diameter, suggesting that these specialized plastids are like tubular chromoplasts. Plastid vacuoles arise from the periphery of dense cores and apparently discharge electron-translucent contents into the suspensor cytoplasm. Plastid vacuoles may play a role in suspensor metabolism and thus influence embryo development.
The genetic diversity and species-diagnostic markers in the introduced apple snail, Pomacea canaliculata and in the native Thai apple snails; Pila ampullacea, P. angelica, P. pesmei, and P. polita, were investigated by restriction analysis of COI and are reported for the first time. Twenty-one composite haplotypes showing non-overlapping distributions among species were found. Genetic heterogeneity analysis indicated significant differences between species (P < 0.0001) and within P. pesmei (P < 0.0001) and P. angelica (P < 0.0004). No such heterogeneity was observed in Pomacea canaliculata (P > 0.0036 as modified by the Bonferroni procedure), P. ampullacea (P = 0.0824-1.000) and P. polita (P = 1.0000). A neighbor-joining tree based on genetic distance between pairs of composite haplotypes differentiated all species and indicated that P. angelica and P. pesmei are closely related phylogenetically. In addition, the 16S rDNA of these species was cloned and sequenced. A species-specific PCR for P. canaliculata was successfully developed with a sensitivity of detection of approximately 50 pg of the target DNA template. The amplification of genomic DNA (50 pg and 25 ng) isolated from the fertilized eggs, and juveniles (1, 7, and 15 d after hatching) of Pomacea canaliculata was also successful, and suggested that Pomacea canaliculata and Pila species can be discriminated from the early stages of development.
We successfully crossed perennial Medicago sativa L. (2n = 4x = 32) and annual M. scutellata (L.) Mill. (2n = 4x = 32). The hybrid was a mixoploid. The primary shoot was hexaploid. Later a tetraploid shoot was identified and another tetraploid shoot had a distinct flower color (dark purple). Somatic chromosome number was unstable, which probably led to chimeral sectoring. Chromatographically the hybrid was documented by species-specific phenolic substances. The hybrid was perennial, but color of the vegetative parts and stem number of the hexaploid sector resembled the annual parent. Flower color of the hexaploid was purple like the perennial; the short peduncle and self-tripping mechanism were like the annual. Leaflet number was unstable, varying from 2 to 6 per leaf. Several characters were intermediate between the parents. Pollen stainability of the hexaploid was low, and no seed was obtained from selfing, backcrossing to either parent, or crossing to Chilean and nondormant M. sativa plants. Other sectors of the mixoploid will be characterized as material becomes available.
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